The present invention relates to an apparatus and a method for bending a glass sheet so that the glass sheet has a compound curvature for use in, e.g., an automobile window.
There are several methods for bending a glass sheet such as an automotive window glass. In one method, a glass sheet is heated to become soft and bent by making the heated glass sheet sag by gravitation in a mold. In another method, a glass sheet is heated and then pressed by press molds. In still another method, a glass sheet is heated and bent while floatingly conveyed over hearth beds in a furnace. More specifically, while the glass sheet is conveyed over a series of hearth beds in a conveyance direction (hereinafter “first direction”), hot gas is ejected toward the glass sheet so that the glass sheet is floated over the hearth beds. The hearth beds have upper surfaces curved along a plane perpendicular to the first direction. The glass sheet is thus heated with the hot gas, gradually sags by gravitation and is bent according to the curvature of the upper surfaces of the hearth beds. In a subsequent stage, the bent glass sheet is quenched in a quenching unit. Alternatively, the glass sheet may be conveyed by a series of rollers in the furnace and bent according to the curvature of the rollers. A decision as to which method should be used is made depending on the size and curvature of bent glass sheets to be fabricated.
Among the above methods, the method using the hearth beds in the furnace (hereinafter referred to as a hearth bed bending method) has the advantage in its productivity for bending the glass sheet along the plane perpendicular to the first direction. Thus, the glass sheets having a simple curvature can be fabricated at low cost by the heath bed bending method. To fabricate the glass sheets having a compound curvature in such a hearth bed bending method, it has been proposed to heat, bend and quench the glass sheets while conveying the glass sheets either upwardly or downwardly relative to the first direction in order to also bend the glass sheets relative to the first direction with a curvature radius of tens of thousand of millimeters. However, it is not easy to bend the glass sheet relative to the first direction while conveying the heated glass sheets because of the furnace structure.
U.S. Pat. No. 3,409,422, corresponding to Japanese Patent Examined Publication JP-B-49-10331, discloses an apparatus in which some hearth beds situated most downstream in a furnace and the lower air blower of a quenching unit are inclined downwardly relative to a first direction to form a curvature curved relative to both of the first direction and a direction (hereinafter “second direction) perpendicular to the first direction, thereby fabricating glass sheets having a compound curvature.
Furthermore, U.S. Pat. No. 6,014,873, corresponding to Japanese Patent Unexamined Publication JP-A-5-9037, discloses an apparatus in which some hearth beds situated most downstream in a furnace are inclined upwardly relative to the first direction so that the final hearth bed and the lower air blower of a quenching unit form an upwardly convex curvature, thereby fabricating glass sheets having a compound curvature. U.S. Pat. No. 5,522,912, corresponding to Japanese Patent Unexamined Publication JP-A-6-191867, discloses an apparatus in which the final hearth bed has a top surface of which at least a downstream part is curved relative to the first direction and is inclined upwardly, for fabricating glass sheets having a compound curvature.
In each of the above conventional apparatuses, the equipment thereof (especially, the final hearth bed) needs to be replaced and adjusted, when different kinds of products are to be fabricated. More specifically, in the U.S. Pat. Nos. 3,409,422 and 6,014,873, the above-mentioned downstream hearth beds and quenching unit need to be replaced with new ones. Then, the newly equipped hearth beds and quenching unit have to be positioned in place by adjusting their respective supports so as to be properly inclined and connected with each other. In U.S. Pat. No. 5,522,912, the final hearth bed has to be replaced with a new one. The newly equipped bed needs to be inclined upwardly by adjusting its support properly. Such replacement and adjustment require much time and labor, whereby the profitable merit of the hearth bed bending method (i.e., high productivity) cannot be maintained. In addition, there arises a problem that the position to take out the bent glass sheets from the apparatus may become too low, when the glass sheets are conveyed downwardly relative to the first direction through the apparatus.
In view of the foregoing, an apparatus has been developed as disclosed in Japanese Patent No. 2809596 (corresponding to Japanese Patent Unexamined Publication JP-A-8-151221) and Japanese Patent Unexamined Publication JP-A-7-237928, in order to save time and labor required to replace and adjust the equipment of the apparatus for fabrication of different kinds of products.
In an apparatus disclosed in Japanese Patent No. 2809596, the final hearth bed is arranged in alignment with other hearth beds for bending glass sheets to have a simple curvature. For bending glass sheets to have a compound curvature, the final hearth bed is rotated about a vertical axis of rotation, without being inclined, so that the upstream end surface of the final hearth bed forms a slight angle with the downstream end surface of a hearth bed next to the final hearth bed, and at the same time, the lower air blower of a quenching unit is inclined downwardly relative to the first direction. In this state, the top surface of the final hearth bed is curved upwardly relative to both the first and second directions, whereby the flat glass sheet is bent into a shape having a compound curvature. Thus, different kinds of products can be fabricated with ease, i.e., a glass sheet can be bent to have a simple curvature or to have a compound curvature with ease because the production line can be modified only by rotating the final hearth bed without replacing it with another one. It is also beneficial that the final hearth bed can be shared even in fabricating different kinds of products. However, when the glass sheet is to be bent with a smaller radius of curvature relative to the first direction, the final hearth bed has to be rotated further so that the top surface of the final hearth bed is curved with a smaller radius of curvature relative to the first direction. In such a case, the space between the final hearth bed and the hearth bed next to the final hearth bed becomes larger, which causes a pressure drop of gas ejection due to gas leak through the space. Buoyancy to float the glass sheet becomes weaker owing to the pressure drop. The glass sheet may be caused to rub against the hearth beds, thereby being formed with flaws and cracks.
Furthermore, in an apparatus disclosed in Japanese Patent Unexamined Publication JP-A-7-237928, one side of a bent glass sheet is quenched, and then, both sides of the bent glass sheet are quenched in order to modify a curvature of the bent glass sheet. It is however necessary to provide additional equipment, such as a gas blowing system for previously quenching one side of the bent glass sheet.
The copending and commonly assigned U.S. patent application Ser. No. 10/119,705, filed on Apr. 11, 2002, discloses an apparatus and a method for bending different kinds of glass sheets, using a final hearth bed having an axis extending between its upstream through its downstream end. This final hearth bed has (a) a top surface curved along a plane perpendicular to the axis with a predetermined curvature radius and (b) an upstream end surface including a first surface section tilted toward a downstream end surface of the final hearth bed so as to form an angle θ to the second direction. The final hearth bed is arranged in a furnace such that the first surface section of the final hearth bed is located adjacent to and substantially parallel to a downstream end surface of the adjacent (penultimate) hearth bed. The above U.S. Patent application discloses another final hearth bed having a top surface curved relative to both the first and second directions so as to be upwardly convex.